Created by John E. Bortle in 2001, the scale helps astronomers and stargazers understand celestial objects’ visibility and the night sky’s quality. Here is a summary of each level:
Class 1: Excellent Dark-Sky Site
• Description: The Milky Way casts visible shadows. Zodiacal light, gegenschein, and airglow are prominent. Constellations and deep-sky objects are easily visible to the naked eye.
• Sky Quality: Truly pristine and dark, with no light pollution.
Class 2: Typical Dark-Sky Site
• Description: The Milky Way is highly detailed. Zodiacal light and airglow are visible. Many deep-sky objects are observable without optical aid.
• Sky Quality: Minimal light pollution, excellent for astronomy.
Class 3: Rural Sky
• Description: The Milky Way is still prominent but not as detailed. Light pollution starts to become noticeable on the horizon.
• Sky Quality: Mild light pollution, good for astronomy.
Class 4: Rural/Suburban Transition
• Description: The Milky Way is visible but lacks detail. Light pollution is more noticeable and begins to affect the visibility of fainter stars and celestial objects.
• Sky Quality: Moderate light pollution, with some impact on stargazing.
Class 5: Suburban Sky
• Description: The Milky Way is faint and appears washed out. Light pollution is evident, limiting the visibility of many stars and deep-sky objects.
• Sky Quality: Significant light pollution, noticeable effect on stargazing.
Class 6: Bright Suburban Sky
• Description: The Milky Way is difficult to see and may not be visible at all. The sky has a noticeable glow from light pollution.
• Sky Quality: Strong light pollution which has a considerable impact on visibility.
Class 7: Suburban/Urban Transition
• Description: The sky is bright with light pollution, making it hard to see most celestial objects without optical aid. Only the brightest stars are visible.
• Sky Quality: Heavy light pollution, poor conditions for stargazing.
Class 8: City Sky
• Description: The sky is very bright due to urban light pollution. Only a few of the brightest stars and planets are visible, and the Milky Way is invisible.
• Sky Quality: Severe light pollution and very poor conditions for stargazing.
Class 9: Inner-City Sky
• Description: The sky is extremely bright, often appearing grey or orange from streetlights. Only the Moon, planets, and a few of the brightest stars are visible.
• Sky Quality: Extreme light pollution, nearly impossible to stargaze.
The Bortle Scale helps amateur and professional astronomers assess the quality of their observing locations and plan their stargazing activities accordingly.
